1. Field of the Invention
The present invention relates to an image processing apparatus and method for compressing/decompressing image data, and more particularly to an image processing apparatus applicable to a copy machine using a printer capable of tone-printing multi-bit image data with continuous tones scanned by a scanner, etc.
2. Description of the Related Art
A conventional digital copy machine stores image data scanned through a scanner in an imagememory, performs editing such as rotation and combining to the data, and reads the resultant data from the image memory to output/print from a printer. For example, with respect to an original placed on a scanning surface with its longitudinal direction parallel to the scanning direction, in the case where recording paper is fed to the printer with its longitudinal direction vertical to the printing direction, the machine is capable of flexibly coping with such a case while rotating the image data scanned from the scanner to suit the recording paper direction.
Meanwhile, a multi-bit printer has been developed which is capable of printing intermediate data as well as binary data. The multi-bit printing enables the image data that has been represented by 1 bit per pixel to be represented by multi-bit data of two or more bits per pixel, and thereby is capable of printing images with continuous tones and of improving the image quality.
When the multi-bit image data is handled, its data amount increases to a few times that of the binary image data, and therefore requires an image memory with a large storage capacity for storing image data, thereby increasing the cost. It is possible to suppress the memory capacity by compressing the image data to store, however, there is a problem that compressed image data cannot be edited depending on the compression system. For example, since the JPEG coding that is a compressing system widely used is of variable-length coding, information on pixel positions is not stored and the editing is difficult. The editing, therefore, should be performed at the former stage of the JPDG coding as illustrated in FIG. 1, however, even in this scheme, the capacity of an editing memory increases and the cost increases. Further, since an editing data amount is large, there occurs a disadvantage that a memory accessing speed needs to be increased not to decrease the processing speed.
In the case of a digital copy machine with both the copy function and facsimile transmission/reception function, when a compression system such as the JPEG system is used which is different from the JBIG system that is one of the compression systems of facsimile standard, the machine needs to be provided with two coding/decoding systems, thereby resulting in problems of increased large circuit scale and increased cost.
In order to cope with such problems, such a method is considered that performs half-tone processing on the multi-bit continuous tone data prior to compression, storage and decompression. As illustrated in FIG. 2, image data is first subjected to half-tone processing to be converted into binary data, and the binary data is rotated by block rotation processing using editing memory 11. Then, the data is subjected to the coding (JBIG coding) of facsimile standard, and the compressed image is stored in image storing memory 12. The compressed image is read from image storing memory 12 to be decoded (JBIG decoding), the decompressed half-tone data is subjected to multi-bit estimating processing to estimate multi-bit image data, and the resultant data is output as multi-bit continuous tone data.
However, since the method for performing the half-tone processing prior to compression, storage and decompression uses a spatial filter with spatial frequency characteristics of a low-pass type as means for estimating the multi-bit image data, there are problems that a blurred portion occurs on a decompressed image, uneven frequencies caused by the half-tone processing are not attenuated sufficiently, and that the image quality deteriorates greatly.